(Not Applicable)
(Not Applicable)
1. Field of the Invention
This invention relates generally to suspension components on bicycles, and particularly to a bicycle seat post which also functions as a shock absorber.
2. Description of the Related Art
The most common design for a bicycle has a quadrangular frame to which a wheel is affixed in the rear and a steering assembly, fork and wheel are attached in front. Just forward of the rear wheel a tubular post, usually called a seat post, is inserted at its lower extremity into a tube in the bicycle frame, and the upper extremity of this post is attached to a saddle on which the rider sits.
On some bicycles, especially those designed for off-road use, shock absorbing components have been incorporated into the frame and front fork. Most commonly these consist of elastomeric springs, coil springs or gas springs, often paired with hydraulic damping means to create shock absorbers similar to those used in motorized vehicles. These shock-absorbing components are usually located in telescoping legs of the front fork and as a spring/shock absorber for the rear part of the bicycle, interposed between the main part of the bicycle frame and a pivoting rear frame member.
Additionally, seat posts have been designed which absorb road shock that would otherwise be transmitted from the frame to the bicycle saddle. Some of these posts consist of two telescoping members with a spring mounted inside and between them in such a way as to resist compression. In others the saddle is mounted to a pivoting linkage which swings down and to the rear while maintaining the saddle in a level position. Between or adjacent to the members of this linkage a spring means is attached in order to absorb jounce and return the saddle and rider to their normal positions.
Shock absorbing seat posts or suspension seat posts lack some of the advantages of shock absorbers integrally incorporated into the bicycle frame. For example, an integrated rear shock absorber can control bouncing and thereby improve traction of the rear wheel even when the rider is pedaling out of the saddle, a function which, by virtue of its location, a suspension seat post cannot duplicate.
Suspension seat posts have their own advantages, however. Unlike integral rear shock absorbers, suspension seat posts need not be incorporated into the bicycle at the design and manufacture stages nor do they require the same relatively expensive mechanical accommodations. Suspension seat posts typically can be manufactured for the same or lower cost as compared with integral rear shocks and can be fitted to a wide variety of bicycles either during manufacture or later as an aftermarket accessory. Functionally, they are capable of absorbing those jounces which are most uncomfortable to the rider and which occur when the rider is in the usual seated position.
In spite of their potential for improving rider comfort and control and for reducing fatigue, in practice suspension seat posts have suffered from design limitations. In order to fit the most widely used type of bicycle frame, seat posts are confined to a maximum diameter that is less than optimal. Seat posts generally must mount at a rearward leaning angle and must prove durable under repeated hard jouncing given a realistic range of rider weights. At the same time there is a general concern with the weight of the bicycle and its parts which limits choice of materials. Additionally, telescoping seat posts must contain a substantial mechanical means for maintaining rotational alignment between inner and outer members given the rotational torque that a rider may generate at the saddle.
These limitations have led designers of suspension seat posts for the most part to employ elastomeric springs manufactured so as to have a relatively linear response to load and sluggish rebound characteristics, and which therefore have no need for a separate damping means. These springs may be used by themselves or in combination with a coil spring to similar effect. While compatible with the space restrictions inherent in seat posts, devices which rely on elastomeric materials are generally acknowledged to be less effective at absorbing jounce and vibration than either coil or gas springs which make use of gas or hydraulic damping.
As noted above, the two members of a telescoping post have a restriction on their outer diameter due to the need for insertion into common bicycle frame tubes. At the same time, a restriction on the relative difference in diameter between inner and outer telescoping members is imposed by strength requirements. Available annular space between the tubes therefore is minimal.
A difficulty arises with respect to the close fit between telescoping members because of two factors. The first of these is the need for rotational rigidity between the members as already noted. The second is the need to provide cushioning for any gas spring upon re-extension after compression, what is commonly called xe2x80x9ctop out.xe2x80x9d The most widely used means for maintaining rotational rigidity in telescoping seat posts consists of two opposed vertical keyway slots on the inside diameter of the lower or outer member. Mating xe2x80x9ckeysxe2x80x9d or plugs fit into opposing cavities on the outside of the upper or inner telescoping member, after which the inner and outer members slide together in such a way that the keys engage the slots. The outer member typically has threads on its outside top end to accept a threaded cap with an inwardly protruding lip that abuts the outside of the inner post member. When the two members are engaged and the cap is screwed down, the members cannot be extended past the point where the keys contact the lip of the cap. This point of contact is effectively the xe2x80x9ctop outxe2x80x9d or extension stop for this type of telescoping design. The only space available to insert a cushion to prevent hard top out contact is the very small annular space between the telescoping members.
In the case of elastomeric springs or elastomer/coil combinations, the amount of extensive force exerted by the spring as it reaches the limit of extension is small. The limited space available is sufficient to contain a means for cushioning top out action when such springs are used. By its nature a telescoping gas spring exerts a much stronger force against whatever mechanical means acts as the extension stop. This force is sufficient to overcome any top out cushioning means located in the space between the two telescoping members.
The top out action of the gas spring may be controlled by making the spring a self-contained mechanical assembly inside the post instead of making it integral to the post. Top out cushioning can then be incorporated internally to this self-contained unit. This requires the use of higher, less practical gas pressures due to the necessarily smaller shaft and piston sections in the spring unit, or else supplementing of the gas spring by putting it in series with a coil spring. This also requires the expense of a separate containment vessel along with additional mechanical parts. Weight is increased, thereby negating part of the advantage of the gas spring design.
An alternate method for maintaining rotational alignment and arresting extension of the telescoping members is to fix a pin or plug of rigid material across the inside diameter of the outer member. This pin traverses an elongated slot extending through the inner member. The slot moves up and down over the pin during compression and extension. The extension stop is the lower extremity of the slot. When the inner member extends upward to the point where the bottom of the slot contacts the underside of the pin, top out occurs. Prior application of this design incorporates no provision for top out cushioning sufficient for use with an integral gas spring.
The need therefore exists for a telescoping seat post that has the performance and weight advantages of an integral fluid or gas-damped gas spring, including sufficient means for cushioning top out on re-extension.
The invention is a seat post for mounting between a frame and a seat of a cycle. The seat post comprises an outer member having a cylindrical bore defined by a sidewall. The sidewall has an inner cylindrical surface and a floor at one end of the bore. An inner member has a sidewall with a cylindrical outer surface and an end forming a piston. The piston extends into the cylindrical bore of the outer member with the outer cylindrical surface of the inner member slidingly, sealingly engaging the inner cylindrical surface of the outer member. This configuration forms a variable volume chamber in the outer member defined by the inner cylindrical sidewall, the piston and the floor. A port is formed in the piston end of the inner member and opens to the variable volume chamber. A fixed volume chamber is formed in the inner member, and is in fluid communication with the variable volume chamber through an axial fluid passage and a restrictive flow orifice formed in the inner member, and both of which are in fluid communication with the port. The fluid, preferably a gas, in the fixed volume chamber can flow into the variable volume chamber only by passing through the restrictive flow orifice, the axial fluid passage and the port.
A check valve is mounted along the axial fluid passage, for allowing at least some of the fluid in the variable volume chamber to pass into the fixed volume chamber without passing through the restrictive flow orifice when the pressure in the variable volume chamber reaches a predetermined threshold. A pair of elongated slots is preferably formed longitudinally in the sidewall of the inner member.